The present invention relates to a switching circuit in a switching-type power conversion apparatus and a method for driving the switching circuit. In particular, the present invention relates to a power conversion apparatus including a switching circuit in which a current control type semiconductor switching element is supplied with a driving power for switching the semiconductor switching element in response to a control signal and a current transformer serving as an auxiliary power supply is connected to the switching element, and to a method for driving the power conversion apparatus.
In view of the efficient utilization of energy, power conversion apparatuses applying a semiconductor switching element are used in significantly wide area due to their excellent property in power conversion efficiency. In the field of such power conversion apparatuses, various approaches have been made to enhance the efficiency. For instance, in the Transactions of the Institute of Electrical Engineers of Japan, vol. 116-D, No. 12, pp. 1205-1210 (1996), it is proposed that a transistor having a small conduction loss is employed as a switching element of a switching circuit, this sort of two transistors being connected to form a Darlington connection wherein the primary one of the two transistor is used as a driving transistor, and a current transformer serving as an auxiliary power supply being interposed between the driving transistor and the switching transistor. This paper reports that the above circuitry may reduce the conduction loss to about one-third as compared to that of conventional switching elements.
According to the circuit described in this paper, a voltage drop in an ON-state of the switching element becomes equal to a saturation voltage of the switching element by virtue of to the auxiliary power supply so that the switching element may be operated with lower voltage drop than that in an ON-state of transistors connected to form an ordinary Darlington-connection, and may provide an improved efficiency. In this case, the current transformer acts to vary a current supplied from the auxiliary power supply in response to an output current of the switching element.
In this conventional circuit, since a driving current of the switching element is defined by a coil ratio of the current transformer, it is necessary for the current transformer to be designed based on a minimum current amplification factor of the switching element. Thus, the switching element can be undesirably driven to run into an oversaturation state during light-load condition, and thereby it is difficult to achieve an optimized conduction loss.
The present invention has been embodied to solve the problem of the aforementioned circuit suggested in the Transactions of the Institute of Electrical Engineers of Japan described above. Thus, it is an object of the present invention to provide a power conversion apparatus capable of reducing the conduction loss to achieve high efficiency yielding a downsized and weight-reduced apparatus and a method for driving such a power conversion apparatus.
In order to achieve the object, the present invention provides a method for driving a switching element in a power conversion apparatus comprising a switching-element driving circuit which includes a current transformer having a primary coil connected to an current control type switching element, and a driving-current generating circuit formed of a secondary coil of the current transformer and a rectifying circuit connected to the secondary coil, wherein an output current generated in the driving-current generating circuit is supplied to the switching element as a driving current of the switching element. This method comprises the steps of detecting an output current of the switching element, and extracting a part of the output current of the driving-current generating circuit out of the switching-element driving circuit so as to variably controlling the driving current of the switching element in response to the output current of the switching element. Preferably, the output current of the driving-current generating circuit extracted out of the switching-element driving circuit is supplied to any other auxiliary power supply as a regenerative power.
In a preferred embodiment of the present invention, a specific value of the output current extracted out of the switching-element driving circuit is determined to substantially achieve an optimum driving current value for minimizing an overall conduction loss considering the conduction loss of the switching element and the driving power necessary for driving the switching element. The driving current of the switching element is preferably variably controlled in response to the temperature of the switching element.
In another aspect, the present invention provides a power conversion apparatus. This power conversion apparatus includes an current control type switching element, a current transformer having a primary coil connected in series with the switching element and a secondary coil, a driving-current generating-circuit formed of the secondary coil of the current transformer and a rectifying circuit connected to the secondary coil, and a switching-element driving circuit supplying an output current generated in the driving-current generating circuit to the switching element as a driving current of the switching element. As features of the present invention, the power conversion apparatus further comprises an output current detecting device for detecting an output current of the switching element, a control device for forming a control signal to variably control the driving current of the switching element in response to a value of the output current of the switching element detected by the output current detecting device, and a current extracting circuit operated in response to the control signal to extract a current having a specific value corresponding to the control signal from the output current in the driving-current generating circuit out of the switching-element driving circuit. Preferably, the current extracting circuit is configured to be a generative circuit for supplying the current extracted out of the switching-element driving circuit to any other auxiliary power supply as a regenerative current. More preferably, this regenerative circuit is adapted to perform power regeneration through a switching operation.
The power conversion apparatus according to the present invention is adapted to detect the output current of the switching element and extract a part of the output current of the driving-current generating circuit out of the switching-element driving circuit so as to variably controlling the driving current of the switching element in response to the output current of the switching element. Thus, the optimum driving current may be supplied to the switching element in response to the output current of the switching element so that the conduction loss of the switching element may be minimized. For example, the driving current of the switching element may be optimized by determining the output current value extracted out of the switching-element driving circuit so as to substantially achieve the optimum driving current value for minimizing the overall conduction loss considering the conduction loss of the switching element and the driving power necessary for driving the switching element.